304 report — 1865. 



surface have been presented to my notice ; and here I would remark that 

 observations of this kind differ considerably from ordinary astronomical 

 observations ; many of the features appear to be exceedingly evanescent, and 

 can only be detected on rare occasions, and these at considerable intervals, 

 the one from the other ; a very slight alteration in the direction of the inci- 

 dent or reflected ray suffices to render an object in-visible which a few hours 

 before might have been seen with distinctness, and good measures of it ob- 

 tained. Webb has alluded to this where he says, in his ' Celestial Objects for 

 common Telescopes,' p. 54, " As the angle of illumination increases, a fresh 

 aspect of things creeps in, and this aspect is such that we may sometimes 

 catch the whole, sometimes a portion, sometimes nothing of many a familiar 

 feature." It is therefore important to seize the opportunity when presented 

 for examining carefully a district the illumination of which is such as to 

 bring out in strong relief the most minute features discernible. During a 

 period of nearly seven years' observations, there are some features that I have 

 seen only four or five times, and some only once. The rill-system on the 

 S.W. of Triesnecker I have seen favourably only on one occasion, nearly 

 twelve months since, and that after waiting many months to obtain it. After 

 carefully scanning the surface of the moon, should an opportunity of the kind 

 now alluded to be presented, no time should be lost, nor trouble spared in 

 obtaining as correct a record as possible of the details visible. The following 

 are a few of the instances in which remarkable features have been brought 

 out with extraordinary distinctness, so much so as to mark out distinctly the 

 boundaries of individual regions. 



The Plain of Dionj/sius. — " On the morning of January 18, 1865, between 

 5 and 7 a.m., the subject of observation was the great Rill of Ariadanis dis- 

 covered by Schroter, and described, under this head, by Beer and Madler. 

 It was well situated, both as regarded visual and illuminating angles ; and I 

 noticed that south of the rill, between Ariadseus and Silberschlag, the surface, 

 which forms a somewhat extensive plain, is slightly depressed, much in the 

 same way as the surface on one side of the Straight Wall is lower than the 

 other, with this difference — the rill of Ariadaeus appears to have resulted 

 from a crack ; and throughout the length above specified the depth of the 

 rill is plainly perceptible by its shadow. The depressed plain south of the 

 rill is extensive ; the four portions of it are symbolized, and in progress of 

 entry in Form No. 3. 



The Valley J. J. Cassini. — " In the course of Mr. Webb's correspondence, 

 that gentleman has communicated an account of the recoveiy of one of 

 Schroter's valleys, named by him J. J. Cassini, which, although appearing on 

 Beer and Madler's map, does not bear the name appropriated by Schroter. 



On the morning of the 9th of February, 1865, I had an opportunity of 

 comparing Schroter's drawing of J. J. Cassini with the moon ; and although 

 the weather prevented that full examination which I desired to make of the 

 region, yet the general accuracy of Schroter's drawing was unmistakeable. 

 The state of libration was such as to bring the objects into full view with 

 less foreshortening ; and although the form of the valley J. J. Cassini was not 

 precisely as Schroter has given it, yet the general outline and the neighbour- 

 ing craters fix Schroter's drawing to the part of the moon which he describes 

 in his text; accordingly Beer and Madler's statement, to the effect that 

 Schroter's drawing is irreconcileable with the surface of the moon, appears 

 to be without foundation *. 



* This statement of Beer and Madler appears to have originated from Schroter mis- 

 naming some craters in the neighbourhood. 



